1. Field of the Invention
The present invention relates to a process for treating aqueous solutions comprising bases and organic acids. More particularly, the invention relates to a process for cleaning process water obtained from aldolization reactions.
2. Discussion of the Background
In many base-catalyzed condensation reactions of organic compounds, in particular, the reaction of aldehydes, carboxylic acids are formed as by-products, which neutralize some of the catalyst. One process of great industrial importance is, for example, the condensation of n-butyraldehyde to form 2-ethylhex-2-enal and water. In this case, the reaction mixture is customarily worked up as follows: In the first step, the product-containing organic phase is separated from the aqueous phase (catalyst solution and reaction water) in a settling tank. Because of the polarity of the organic acids formed in the side reactions and of the base used as catalyst, these two components are principally found in the aqueous phase. Direct feed of this aqueous phase to the effluent treatment plant is, therefore, frequently undesirable, because of the high concentrations of bases and organic constituents. In addition, because of the injection of the base from the process together with the aqueous phase, in order to maintain a constant catalyst concentration, fresh base must constantly be fed to the process, which is cost-intensive and contradicts the actual meaning of catalyst.
WO 92/07648 discloses a process for recovering sodium hydroxide solution from process waters of the above-mentioned type. In this process, the object of which is to treat the aqueous phase which is produced by the aldol condensation, cation-exchange membranes are used. The aqueous phase in this process comprises the sodium salts of the organic acid or alcohol formed as by-products of the aldol reaction. This solution is passed through the anode chamber in a two-chamber membrane electrolysis cell. When an electric field is applied, the metal cations migrate into the cathode chamber and react with hydroxide ions which form at the cathode to form sodium hydroxide. In this process, only the sodium hydroxide solution is separated for use as catalyst. In the anode circuit, an aqueous solution remains which predominantly comprises only the organic acid or alcohol. Disposal of this solution is complex, since the organic constituents are only present in low concentrations and the solution thus cannot be fed directly to the thermal disposal system. Recovery of the organic acids present in low concentrations is also not worthwhile.
DE 196 04 903 discloses a process for separating alkali metal hydroxide solution from the product formed in the aldol condensation, which process operates according to the principle of the three-chamber electrolysis or electrodialysis. In this process, the product-containing phase, after separation of the aqueous phase, is passed through the central chamber of the electrolysis cell, which is bounded by an anion-exchange membrane and a cation-exchange membrane. In the electric field, in the case of sodium hydroxide solution, the Na.sup.+ ions migrate through the cation-exchange membrane and form, with the OH.sup.- ions produced at the cathode, sodium hydroxide solution in the cathode chamber. The OH.sup.- ions present in the central chamber migrate through the anion-exchange membrane into the anode chamber and react with the H.sup.+ ions produced at the anode to form water. In this process, in order to separate sodium hydroxide solution, the base is moved from the product, but the base present in the aqueous phase and the organic constituents are not worked-up.
In the two above-mentioned processes, the phase to be treated is depleted in the course of the electrolysis or electrodialysis in dissolved metal hydroxide, and the conductivity of the solution decreases with advancing electrolysis or electrodialysis. This means that a constantly increasing power input becomes necessary to separate constituents still present in the aqueous solution.
Another process which is known for treating aqueous solutions comprising organic constituents is acidifying the solution until phase separation occurs. However, in this case, only the high-boilers are removed from the solution, while organic acids remain in the aqueous phase. In addition, further salting of mass streams of the aqueous solution is performed, which makes disposal more difficult.